We are looking for lines and there are many of them in this photo. The majority of the surface is flat with some fine parallel lines showing. My question to each of you: is the area within the triangle a raised area projecting towards you or is it a depression projected away from you? View it full size, copy it so you can turn it, enlarge it, study it in detail.
Now for the rest of the story. This is a vertical surface in a granite quarry. The rock has been cut using a diamond wire saw – thus all the fine lines from upper-right to lower-left. Within the granite there are a number of small fractures, two of them being somewhat vertical and one in a more horizontal plane. The three fractures intersect beyond the plane of the cut. When the cut was finished and the stone, where I am standing, was removed, this small triangle of stone fell out of the remaining wall rock.